This invention relates to a method of producing a zinc oxide varistor and, more particularly, to a method of readily producing a zinc oxide varistor having excellent electrical properties and improved stability for DC (direct current) stress.
Zinc oxide varistors are polycrystaline ceramics which exhibit highly non-linear current-voltage characteristics, and widely applied to home appliances, factory devices, power transmission lines and other many kinds of instruments having electrical circuits, to protect them from damages to power surges by taking advantage of the nonlinearity between current and voltage of a varistor. The relationship between the current and voltage of a zinc oxide varistor is expressed by the following empirical equation: EQU I=(V/C).sup..alpha.
where I is the current flowing through the varistor, V is the voltage applied to the varistor, C is constant and .alpha. (alpha) is a non-linear coefficient greater than 1, and is a measure of the non-linearity of the resistance characteristic of the varistor. It is generally desired that alpha is relatively high. Alpha is calculated according to the following equation: EQU .alpha.=log(I.sub.1 /L.sub.2 /log(V.sub.1 /V.sub.2)
where V.sub.1 and V.sub.2 are the voltages at given currents I.sub.1 and I.sub.2, respectively. I.sub.1 and I.sub.2 are generally determined at 1 mA and 10 mA, respectively, and V.sub.1 is called a varistor voltage.
Zinc oxide varistors are usually produced as follows:
A plurality of additives are mixed with a powdered zinc oxide. Typically, 4 to 12 additives are employed.
The types and amounts of additives employed vary with the properties sought in the varistor. The additives are usually metal oxides such as Bi.sub.2 O.sub.3, CoO, MnO, Sb.sub.2 O.sub.3, Cr.sub.2 O.sub.3, SnO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2 and SiO.sub.2. In some cases, metals and metal halides, which are converted to metal oxides by firing under the air, are also used as additives instead of the metal oxides. The amounts of additives are usually very small as compared with zinc oxide. In most cases, amounts of all together additives are less than 5 to 10 mole % of the mixture of additives and zinc oxide.
A portion of the zinc oxide and additives mixture is then pressed into a body of desired shape and size.
Next, the body is sintered at appropriate temperature.
Subsequently, the sintered body is attached with electrodes and leads, then encapsulated by conventional methods. Thus, a varistor is formed.
The conventional methods for the production of a zinc oxide varistor suffer from a serious problem.
That is to say, the properties of a varistor would widely vary, which make it impossible to efficiently produce varistors of constant properties. This problem might be caused by the fact that there are many kinds of additives to be used and these additives are not mixed uniformly with zinc oxide powder as well as each other.
Furthermore, it is difficult to keep the respective purity and particle size distribution of so many kinds of additives in the constant ranges from lot to lot.
Thus, it is highly difficult to uniformly control the microstructure and the micro distribution of chemical components of the varistor comprising many components at high reproducibility.